Such prepregs are used when making electrical insulation for transformer windings and as a rule consist of a substrate, for instance a fiberglass fabric impregnated with a thermosetting synthetic resin.
When manufacturing said insulation, these prepregs are deposited for instance in tape form on a metal strip and while applying high tension are wound on a mandrel which then is moved into a heated oven. The prepreg synthetic resin in the B stage is then converted at these higher temperatures into the C stage. However, this metal-prepreg compound being under high tension, it may happen, especially where the metal sites are uneven, that the resin will flow out of the fiberglass fabric before curing, as a result of which unimpregnated layers of fiberglass will be present between the metal layers. In that case, shorts will occur at the first use of the transformer windings.